Flavonoids are ubiquitous, naturally occurring polyphenolic compounds that are often responsible for the bright, attractive colors of plants. Concentrated in numerous fruit, vegetables, berries, grains, roots, stems, and also in beverages like tea, coffee, beer and wine, they are taken up with the diet, and are eventually deposited in the living human tissue cells. Flavonoids have generated enormous interest due to their obvious benefits for human health. One motivator is the explanation of the “French Paradox”, which is a surprisingly low cardiovascular mortality rate observed in Mediterranean populations, in spite of the relatively high saturated fat intakes. There is compelling evidence now that certain flavonoids present in red wine, which is consumed in relatively high concentrations along with the fat intakes in Mediterranean diets, are indeed responsible for this effect [1]. Probably based on their common antioxidant function, other kinds of flavonoids present in different food sources appear to have a wide range of beneficial effects as well. They have been associated with the scavenging of free radicals, the prevention of DNA damage, protection from UV-light induced tissue damage, the regulation of good and bad cholesterol levels, clearing of arteries, blocking of tumor growth, the promotion of weight control, protection of retinal pigment epithelial cells from oxidative-stress induced death, etc. [2, 3]. Epidemiological studies consistently show that the consumption of flavonoid-rich food lowers the risk of cancers anywhere from 30 to 75% [2].
The molecular structure common to all flavonoids includes two aromatic benzene rings on either side of a 3-carbon ring skeleton, C6—C3—C6, as illustrated in FIG. 1. Depending on the position of carbon double bonds in the C3 ring, substitution of an OH side group and/or double-bonded oxygen, flavonoids are divided into six main categories. These are flavonols, flavones, flavanones, catechins, isoflavones, and anthocyanidins, all shown in FIG. 2 along with selected representatives and major food sources.
Flavonoid categories with most compounds are flavonols and flavones, both of which have a planar structure due to a double bond in the central C3 ring. The most prominent and probably the most investigated members are quercetin and kaempferol, found in high concentrations in onions, broccoli, apples, and berries. The third flavonoid category, flavanones, is mainly found in citrus fruit. Members of this group are naringenin and hesperetin. A fourth category, catechins, is mainly found in green and black tea and in red wine, while the fifth category, isoflavones, is relatively narrowly distributed in foods, with soy beans being the primary food source. The last category, anthocyanidins, is dominant in cherries, berries, and grapes. Synthesized by plants, flavonoids are often bound to other molecules, such as sugars, in this case forming an inactive glycoside complex. The sugar group is known as the glycone, and the non-sugar group as the aglycone or genin part of the glycoside. As an example, citrus fruit contains hesperidin (a glycoside of the flavanone hesperetin), quercitrin, rutin (two glycosides of the flavonol quercetin), and the flavone tangeritin. In living organisms, like in the human body, enzymes can break up the inactive glycosides if needed, and the sugar and flavonoid components are then made available for use.
Relatively little is known about the energy levels of flavonoids except that the strong electronic absorption transitions connecting these levels occur at relatively high optical energies in the deep WV to blue spectral region. In flavones and flavonols, two characteristic absorption bands have been described in the literature: a “long-wavelength” band in the 300-400 nm region, mostly representing the B-ring absorption, and a “short-wavelength” band in the 240-280 nm region, mostly representing the A-ring absorption. Absorption line shapes and strengths of specific flavonoids are thought to depend on the specific number of hydroxyl groups and/or other substitutions as well on their relative positions [4, 5]. For example, comparing the flavonols quercetin and kaempferol with the flavones luteolin and apigenin, it was found that the two flavonols both have a slightly larger (˜30 nm) red shift of their long-wavelength, B-ring, absorption bands relative to those of the two flavone members [6]. This was attributed to the fact that the two flavonols have a hydroxyl group attached to their C3 ring, while the two flavones have no such attachment. For quercetin, the main observed absorption transitions, i.e. those with high oscillator strengths, have been fairly accurately modeled in quantum-chemical configuration interaction calculations, taking into account all excitations from the nine highest occupied molecular orbitals to the nine lowest unoccupied molecular orbitals [7, 8]. The absorption band in the 300-400 nm range is shown to be primarily due to a transition between the highest occupied and lowest unoccupied π molecular orbitals, respectively, where the electronic charge density is withdrawn from the B ring to the C═O double bond of the C ring. The transition in the 240-280 nm region is assigned to a transition between the second highest and lowest π molecular orbitals, respectively, involving a charge transfer from the region of one aromatic ring through C to the other aromatic ring. No information is given on the existence of energy levels, associated charge distributions, and potential low-energy transitions that could give rise to absorption bands on the long-wavelength side of the B-ring 300-400 nm absorption.